1) A first capture with short exposure, potentially with very high ISO, to capture the stars as points. I'll later clean the noise.
2) A second capture with low ISO and long exposure to capture the details of the scene.
3) Mix the long exposure and the stars together.

The problem that I face is the very strong contrast between the scene (second capture) and the sky (first capture). Photomatix's HDR does not work well, neither photoshop's one.

Manually, the problem is that selecting the sky from the sort exposure shot is tricky, especially between trees. The result is a failed picture where the merging line is very noticeable and unnatural.

4 Answers
4

You could try the "Blend If" sliders in your layer options. This ought to blend in the sky within the trees, but it may block out the stars.

Better control would be to create a mask using Image > Apply Image. Once you have that mask (which will have grey tones, use levels or curves on the mask to increase the contrast of the dark sky areas until the mask is black/white. You should be able to clearly define the sky against the trees. If you are left with "holes" where the stars are, just paint over those with a brush. You might also need to feather the mask a bit.

If done right there should be very little difference between the short and long exposures, other than noise. Basically the difference in light levels between the short and long exposure should be evened out by the ISO amplification.

You may have to tweak the camera settings or raw conversion to get the image brightness to match, but once it does you can just align the layers then paint in the low noise foreground using layer masks.

I'm not having luck this approach. The bright image has almost white sky and good land, whereas the stars image has the land black. Combining this black/white borders is not working for me.
–
JavierOct 10 '12 at 16:17

How about layering the star picture on top of the long exposure picture, and use the blending style "lighter color". If the noise is sometimes brighter, lower the brightness of the star image so only the stars themselves are brighter.

I do a lot of night photography and have found that EasyHDR (http://www.easyhdr.com/) gave good results, though I often had to play with it and use blending options other than "true HDR" or the result was too noisy. I don't find the auto-alignment to be as good as Photomatix but if I'm willing to do that manually, I always get good results from it.